In adult brain, it is a long established theory that a new neuron is not generated, and that in an injury to the central nerve system caused by brain infarction, brain hemorrhage, spinal cord injury, etc. and in a neurodegenerative disease such as Parkinson's disease and amyotrophic lateral sclerosis (ALS), recovery of the function which the movement was lost according to the cell death of a neuron was difficult. In recent years, however, it was shown that a neuron was newly generated in adult brain (hippocampus, cerebral cortex association area, lateral cerebral ventricle) of higher mammals such as human and monkey, and that a new neuron in these regions was generated from a neuronal stem cell. It was also demonstrated that a neuronal stem cell existed in aged people's brain and it could differentiate into a neuron. These facts suggest that cerebral regenerative medical treatment is not limited to cell therapy which transplants cells, but therapy which activates inherent neuronal stem cells directly by administrating a drug medicine containing protein or compound or by gene therapy technology is possible.
In order to obtain a specific polypeptide or a cDNA encoding it, there have been generally employed methods comprising confirming the target biological activity in a tissue or a cell culture medium and then cloning of a gene through the isolation and purification of a polypeptide and further methods comprising expression-cloning of a gene with the guidance of the biological activity. However, it is frequently observed that a gene, which has been cloned with the guidance of a certain activity, codes for a known polypeptide since many physiologically active polypeptides occurring in vivo have various biological activities. Further, most intravital physiologically active factors are generated only in a trace amount or under a specific physiological condition, which makes the isolation and purification thereof and the confirmation of biological activity difficult.